Evaluation of critical shattering time of early-maturity soybeans under early soybean production system

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1 AGRICULTURE AND BIOLOGY JOURNAL OF NORTH AMERICA ISSN Print: , ISSN Online: , ScienceHuβ, Evaluation of critical shattering time of early-maturity soybeans under early soybean production system Lingxiao Zhang 1 and Steve Boahen 2 1 Delta Research and Extension Center, Mississippi State University, Stoneville, MS 38776; 2 International Institute of Tropical Agriculture (IITA), Av. Eduardo Mondlane 326, CP 709, Nampula, Mozambique. *: Correspondent author: lzhang@drec.msstate.edu; Tel: (662) ABSTRACT One of the major problems associated with the early soybean production system (ESPS) in the Midsouth USA is seed shattering of early maturity group (MG) soybean that mature in the midsummer. Information is needed to measure the impact of this problem and to provide proper management strategies. Studies were conducted to investigate the problem of shattering in MG IV soybean, the dominant soybean group in ESPS, in 2006 and The objectives of this study were to determine the pattern and critical period of seed shattering of MG IV soybeans under various climatic and production conditions in the Mississippi Delta. A total of 56 and 80 MG IV soybean varieties were evaluated in the experiments in 2006 and 2007, respectively. The varieties were all selected from a Mississippi Soybean Variety Trial and the study was carried out at Stoneville Mississippi. In 2006, only the April planting (April 19) under irrigation was investigated. In 2007, experiments were conducted on both irrigated and non-irrigated fields. On the irrigated tests, both April (April 23) and May (May 15) planting were examined. Results from both years have indicated that most pods of early MG IV soybean varieties can hold seeds relatively well for the first three weeks after maturity (WAM). However, differences were noted starting from the fourth WAM. Non-irrigated soybean shattered faster than irrigated soybean after three weeks. Irrigated soybean held seeds longer than non-irrigated soybean during the fourth week; however, seed shattering became greater after four weeks even in the irrigated study. When comparing early- and late-planted soybean under irrigated conditions, the later maturing pods held seeds better within the same period after maturity (up to 6 weeks or longer). Latematuring pods tended to held seed better, most likely due to lower temperatures experienced after late September. Keywords: Soyabean, shattering time, early maturity INTRODUCTION As a result of the adoption of the early soybean production system (ESPS) in the Mississippi Delta, maturity group (MG) IV soybean has become popular among soybean growers in the region in recent years. In the ESPS, MG IV soybean is usually planted in April and matures from the middle of August to early September (Zhang et al., 2004), when temperatures are still high. Many soybean growers also grow and harvest other crops, such as corn, rice and grain sorghum, at the same time. Delaying harvest due to either manmade factors or natural unfavorable weather conditions is a common problem, which is often causing seed shattering. Therefore, seed shattering is one of the major concerns for local growers to grow MG IV soybeans. Delaying soybean harvest often causes considerable yield lost for soybean producers (Philbrook and Oplinger, 1989; Tukamuhabwa et al., 2002). Previous studies indicated that there were significant differences in shattering resistance among different varieties (Caviness, 1969; Misra et al, 1980) and the characteristic of seed shattering is genetically controlled (Bailey et al., 1997; Caviness, 1969; Saxe et al., 1996). About two-third of the soybean in Mississippi are grown on nonirrigated fields (Zhang et al., 2001), which may have different scenarios in regarding to seed shattering and delayed harvest. However, many growers in the Mid-south that grow irrigated soybean also grow rice that matures at an approximate time as that of MG IV soybean. The conflicting harvest times between soybean and rice are obvious. Limited information is available on the

2 critical shattering period under different circumstances and the relationship between delaying harvest and seed yield lost under the ESPS. Growers need sufficient information to make a proper harvest decision. Therefore, it is important to investigate the critical shattering time for MG IV soybeans under the ESPS. It is also very important for us to know whether a significant difference is existed among the varieties in shattering-resistance characteristic. This information will assist soybean growers in making their decision regarding to variety selection. The objectives of this study were: 1) to determine the critical shattering period(s) for MG IV soybean varieties grown under both irrigated and nonirrigated conditions in the Mississippi Delta; 2) to investigate if timings of planting and harvest alter the critical shattering period, and 3) to evaluate the effect of seed shattering on final yield and economic impact of goring MG IV soybean. From this study, we hope to identify the varieties that are more resistant to seed shattering so that they can be used to fit in the ESPS. This knowledge will help Delta growers to conduct best management practices and obtain the highest yields. MATERIALS AND METHODS Planting, Variety and Experiment Arrangement: This study mainly focused on MG IV soybean since this group of soybean was mostly used for the ESPS, but had a relatively shorter shattering life compared with to that of later MG (such as MG V) soybean. Sixty three MG IV soybean varieties were originally planted on an irrigated field on April 18, Seven varieties were eliminated prior to harvest due to maturity date consideration (either too early or too late). The maturity dates of selected varieties ranged between September 4 and 11 with an average maturity date of September 8. These plots were part of the Mississippi Soybean Variety Trials (MSVT) at the Stoneville location. Plots were planted with standard procedures established for MSVT (White et al., 2006). These standards were for a 4-row plot, 16 ft in length and 20-inch row spacing. Interplant space was approximate two (2) inches (~ 6 plants per foot). The experiment was arranged in a complete randomized block design with three replications. In 2007, a 2-row plot was used with total 80 varieties on two fields, irrigated and non-irrigated. On the irrigated field, two planting dates, April 17 and May 9, were included. The plot length, interplant distance, and experimental design were the same as that of the previous year except that four replicates were used. Data collection, adjustment and analysis: In 2006, the middle two rows of the plots were harvested for yield. Rating of shattering was conducted on the remaining two side rows on October 5 th, 2006, approximately four weeks after average maturity date of the tested varieties. Visual ratings for shattering were conducted using the scale and criteria outlined in Table 1. All three replications were examined for each variety. It was noticed that most pods started shattering from the top (Fig. 1) and then progressively downward to the lower part of the plant. Therefore, this phenomenon was used as one of the measurement criteria in the determination of shattering score. From field observations in 2006, it was noticed that the degree of shattering increased one scale level for every two days during the period of four weeks after maturity. Since the observation of shattering was made at the same day for all the varieties that matured at different dates, the final shattering rates/scores were adjusted based on actual maturity dates of the individual variety. The adjustment was made based on the following procedures: 1) If a variety had a maturity date between September 7 and 9 (within one day before or after the average maturity date, September 8), then the actual rating score was used; 2) If the date was between September 10 and 11 (2-3 later than the average maturity date), the rating score was moved up to the next shattering scale; 3) If the date was between September 4 and 6 (2-4 days earlier than the average maturity date), the rating score was moved down to the next shattering score. For example, if a variety matured on September 6 and was scored at 15% shattering, the adjusted score was finally set at 10% since it matured two days earlier than the average date, September 8 th (Table 1). With this correction, the adjusted shattering score should be more accurate than the direct recorded scores. In 2007, maturity dates of 80 MG IV varieties were recorded. The variety shattering status was visually estimated at weekly interval (based on the criteria described in Table 1) starting from two weeks after maturity (WAM) until five to six WAM. The shattering scores were also adjusted accordingly. Due to the fact that only two rows available in a plot, yield data were collected directly from the experiment plots; however, yield data from the 2007 Mississippi State Soybean Variety trial 441

3 plots at Stoneville location were used as the reference of yield potential for each variety (Table 2). Since the varieties and growth conditions were different in two years, data were analyzed and reported separately. The adjusted shattering score for each variety reported here is the mean of three and four replications in 2006 and 2007, respectively. RESULTS AND DISCUSSION Field observation from 2006 indicated that most pods of MG IV soybean could hold seeds well for the first 3 WAM. However, differences among varieties were observed during the fourth week (Fig. 2). Among the 56 varieties examined in 2006, pods of 25 varieties (or 44.6%) still held seeds very well (shattering scores of 5% or less, 12 varieties) or relatively well (shattering scores of 10%, 13 varieties) at the end of 4 WAM. However, 31 varieties (about 55.4%) had shattering score of 15% or greater. The average shattering score of the examined varieties at the four WAM was 13.3% with a range of shattering scores from 0% to 40% (Table 2). A regression analysis was conducted between shattering percentage at 4 WAM of the tested varieties and the final seed yields of the correspondent varieties MSVT of the same year, indicating no significant correlation existed between the two data sets. This should be primarily due to the timely harvest (before 4 WAM) of the plots of MSVT in Therefore, it indicates that shattering behavior of a variety has no relationship with its yield potential if the seeds can be harvested on time. Results of 2007 were similar to those in On the non-irrigated field, 88.8% of varieties held seeds very well (shattering rate at 5% or less, 30 out of 80) or relatively well (shattering rate at 10~15%, 41 out of 80) at 3.5 WAM (Table 3). However, after 4.5 WAM, only 11.3% (9 out of 80) varieties held seeds relatively well (shattering rate at 10% or less, Table 3). Non-irrigated plots had a faster shattering rate after 4 WAM (Fig. 3); meanwhile, late planted soybean on irrigated fields did not have a severe shattering problem after 4 WAM likely due to later maturity at lower temperatures in late growing season. On the irrigated field, about 18.8% (15 out of 80) varieties held seeds very well at 4 WAM. However, there were 23 varieties (28.8% of 80) that had more than a 15% shattering rate. Above results indicated that 4 WAM is a critical point for early soybean in terms of shattering. A regression analysis was conducted between shattering scores and final seed yields for the irrigated MG IV soybeans at 4 WAM using data from MSVT plots at the Stoneville location, which was planted one day later. Results were similar to that of 2006, which indicated no significant correlation between shattering score and final seed yield when seeds were harvested on time. Comparing overall seed shattering patterns between irrigated and non-irrigated fields, pods were similar in holding seeds for the first three to four weeks, which showed limited shattering. However, non-irrigated soybeans shattered in a faster rate than that of irrigated soybeans after four weeks (Fig 3). Irrigated soybeans held seeds well within four weeks. The seed shattering rate also accelerated after four weeks, but the shattering rate was not as fast as that of the non-irrigated (Fig. 3). Comparing early and late planted soybeans on the irrigated field, pods of the late planted soybean held seeds better after maturity, even up to more than six weeks (Fig. 3). Later maturing seeds tended to have less shattering, most likely due to lower temperatures after late September. Although there was no direct relationship between shattering and final yield when seeds were harvested in time, it is expected to have yield impact if harvest is delayed and shattering rate reaches 15% or more. Meanwhile, in real practice, harvest may often be delayed due to environmental or man-made reasons, such as unfavorable weather conditions, labor arrangement and/or machinery shortage. Therefore, a variety with a better shattering score should have the advantage in reducing the yield loss when harvest is delayed. A list of varieties with good shattering resistance (shattering score was at 10% or less at 4 WAM) is provided in Table 4. Quantitative measurement of shattering impact on yield lost was not performed in this study. However, it is an important parameter to be focused in future breeding study, especially for those soybean varieties adapted to the Midsouth climate under ESPS conditions.. 442

4 Table 1. Estimation and description of the percentages of soybean shattering status in a plot. A single line plot is used and the plot length is 16 feet long with 2.5 inches plant-to-plant distance. Scale % of shatter Criteria and observational description 1 0 Absolutely no shattered pod 2 2 Five or less shattered pods are observed in a plot 3 5 More than five but less than 15 shattered pods are observed and they are mostly on the top of the plants 4 10 More than 15 shattered pods are observed, but it is estimated at 10% or less of whole pods in the plot. A few shattered pods are in the upper middle or middle of the stems 5 15 Many top pods are shattered and some of the lower portions of the pods are also shattered. This is considered as a critical stage that has a significant effect on seed yield Almost all the top pods were shattered, and shattered pods on 1/4 of the plants. Pods shattering more than 25% results in severe yield loss All top pods shattered, and the shattered pods went down to the middle part of the plant Most of the pods are shattered Table 2 Summary of adjusted shattering score (ASS) and yield of MG IV soybean varieties at Stoneville, MS at four weeks after maturity. Planting date was on April 18, Variety ASS (%) 1 (bu/a) Variety ASS (%) 1 (bu/a) Yield Yield DG 4660RR DPX 4112RR/S DK AG DG37F DG37A V47N6RR RC USG 7466nRR Armor GP MorSoy RT 4665N RC DG 32R Progeny 4406RR AG AG Progeny 4401RR DK Progeny 4206RR DG 35B DG 3443NRR Armor GP MorSoy RT Progeny 4216RR DKB USG 74T HBK HX USG 74C DK XTJ RR DP 4546RR AG DK 4661RR Progeny 4405RR DK XTJ HBK R NK S43-B DG 3463NRR Progeny 4506RR AV44D TV 46R V44N6RR RR XG USG 744nRR DBK DP4331RR TVX 44R M RC NRR AV46JJ5NRR MorSoy RT 4485N RC M HBK R Average LSD (α = 0.05) Shattering score was determined according to the scale in Table 1. Since the evaluation of shattering status for all varieties was scored at the same date, the adjusted shattering score is calculated based on the maturity dates of individual varieties. See details for the adjustment procedures in Materials and Methods section of this paper. 443

5 Table 3. Comparison of number of varieties with each shattering rate category around fourth week after maturity (WAM) for MG IV soybeans grown under non-irrigated and irrigated field conditions in the Mississippi Delta in Number of total variety is 80. Number listed in the table is number of variety, and the number in parenthesis indicates the percentage of total variety. Harvest treatment (WAM) Non-irrigated Irrigated Shattering score (%) % 30 (37.5) 1(1.3) 15 (18.8) 6-15% 41(51.2) 8(10.2) 42 (52.5) >15% 9 (11.3) 71 (88.5) 23 (28.8) Table 4. List of varieties (alphabetical order) that were holding seeds well at the fourth week after maturity (shattering at 10% or less) in Yield potentials are listed for reference Variety Yield (bu/a) Variety Yield (bu/a) AG DK AG DK AG DKB AG DP 4546RR 49.3 AV49J7NRR 54.5 DP 4724RR 57.4 DG 32R HBK R DG 35Z NK S43-B DG 36Y NK S46-U6 --- DG 37P P4206RR 40.7 DG 38X P4405RR 55.6 DG 4770RR 54.8 P4606RR 58.1 DG 4780RR 52.7 P4906RR 37.7 DG 4840RR 47.3 RC DG 4860RR 62.1 RC DG 4970RR 56.1 RT4485N --- DG 4975LARR 58.0 RTS4955N --- DG4960RR 55.6 TV44R DK TV48R DK USG 7466nRR 50.6 Average 53.6 LSD (α = 0.05)

6 Fig 1. Photo demonstration of seed shattering showing the process starts from tip of the soybean plant. Number of vareity Precentage of shattering Fig 2 Distribution of number of varieties with various percentage of shattering at 4 weeks after maturity in Total 56 varieties are included in the experiment. 445

7 Fig 3 Comparison of shattering patterns among three growth conditions: non-irrigated (NI), irrigating early (IE) and irrigated-late (IL), under early soybean production system in Mississippi Delta in Data in the figure are the average of 80 varieties used in the experiment. The date with a shattering score of 0 % in each pattern indicates the soybean maturity date of the corresponded growth condition Yield Shattering percentage Fig. 4 Relationship between shattering percentage at 4 weeks after maturity and final seed yield. No significant relationship was found. CONCLUSION Maturity group IV soybean grown in the Mississippi Delta under ESPS conditions has a critical shattering period, which occurs at about the fourth week after maturity, but could be earlier depending and non-irrigated fields. Meanwhile, there are significant differences existing among the varieties, which can be considered as one of the criteria for variety selection. Information from this study indicates that soybean should be harvested before on the weather conditions. Also, soybean four weeks of maturity in the MS Delta, especially shattering patterns are varied between irrigated for non-irrigated soybean under ESPS. Soybean 446

8 that matures after mid-september may hold for 5-6 weeks after maturity; therefore, they may be harvested at a later time if labor and other arrangements are considered. ACKNOWLEDGEMENT This research was supported by Mississippi Soybean Promotion Board check-off fund REFERENCES Bailey, M.A., M.A.R. Mian, T.E. Carter, D.A. Ashley & H.R. Boerma, (1997). Pod dehiscence of soybean: Identification of quantitative trait loci. J Hered 88(2): Caviness, C.E., (1969). Heritability of pod dehiscence and its association with some agronomic characters in soybeans. Crop Sci 9: Misra, R., Singh, S. Sharma S. and Mehta, S. (1980). Note on induced variation in shattering habit of soybean. Indian J Agri Sci 51(9): Philbrook, B. and Oplinger, E.S. (1989). Soybean field losses as influenced by harvest delays. Agronomy Journal 81: Saxe, L.A., C. Clark, S.F. Lin and T.A. Lumpkin, (1996). Mapping the pod-shattering trait in soybean. Soybean Genet Newsl 24: Washington State University, USA. Tukamuhabwa, P., K.E. Dashiell, P. Rubaihayo, and M. Nabasirye. (2002). Determination of field yield loss and effect of environment on pod shattering in soybean. Afr Crop Sci J. 10 (3): White, B, A. Blaine, B. Burgess, P. Gerard, W. Maily, R. Martin, D. Poston, T. Rector, A. Smith, C. Watson, M. Young, and L. X. Zhang Mississippi Soybean Variety Trials, Mississippi Agri. & Forest Exp. Stn. Info. Bull. 425 Zhang, L.X., J. Zhang, S. Kyei-Boahen, and C. E. Watson, Jr. (2004). Developing phenological prediction tables for soybean. Online. Crop Management. doi: /cm rs. Zhang, L, D. Poston and F. Cooke. (2001). Survey of soybean production system in Mississippi Delta. (Abstract). In 2001 Annual Meeting Abstracts. ASA/CSSA/SSSA, Madison, WI. 447